Dehydrogenative Aromatization and Sulfonylation of Pyrrolidines: Orthogonal Reactivity in Photoredox Catalysis

Article abstract of DOI:10.1002/anie.201808427

Oxidative dehydrogenative aromatization and selective sulfonylation reactions of N-heterocycles under visible-light photoredox catalysis were established. The mild reaction conditions make this approach an appealing and versatile strategy to functionalize/oxidize pyrrolidines whereby arylsulfonyl chlorides were identified to be both catalyst regeneration and sulfonylation reagents.

Full text of DOI:10.1002/anie.201808427

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International Edition: DOI: 10.1002/anie.201808427
German Edition: DOI: 10.1002/ange.201808427
Photoredox Catalysis
Dehydrogenative Aromatization and Sulfonylation of Pyrrolidines:
Orthogonal Reactivity in Photoredox Catalysis
Krishnamoorthy Muralirajan+, Rajesh Kancherla+, and Magnus Rueping*
Abstract: Oxidative dehydrogenative aromatization and selec-
tive sulfonylation reactions of N-heterocycles under visible-
light photoredox catalysis were established. The mild reaction
conditions make this approach an appealing and versatile
strategy to functionalize/oxidize pyrrolidines whereby arylsul-
fonyl chlorides were identified to be both catalyst regeneration
and sulfonylation reagents.
The use of readily available photosensitizers, catalysts, light
sources and the mild reaction conditions allows the efficient
generation of reactive intermediates which lead to a diverse
range of synthetically useful molecules from feedstock sub-
strates. In addition, new photoredox protocols circumvented
the use of stoichiometric oxidants resulting in more sustain-
able synthetic transformations.[7] However, most of these
protocols are based on the combination of a photoredox
catalyst and metal catalyst or the use of metal nanoparticles.
For instance, binary hybrid catalytic systems consisting of
a photoredox and palladium or cobalt catalyst were recently
reported for acceptor-less dehydrogenations.[8] Additionally,
an efficient Pt@TiO2 catalyst was described for the mild
dehydrogenation of cyclohexanes.[9]
In continuation of our efforts to develop green and
efficient visible-light photoredox reactions without the need
of additional transition metals,[10] we recently observed an
unusual aromatic product formation. Herein, we would like to
report our observation which lead to the development of
a new visible-light photoredox-catalyzed synthesis of b-
substituted pyrroles from unactivated pyrrolidines via an
electron acceptor, metal-oxidant-free dehydrogenative aro-
matization reaction (Scheme 1).
Oxidative dehydrogenation of saturated N-heterocycles to
aromatics is a promising method with great use for chemical,
biological, and pharmaceutical applications.[1] Generally, the
oxidative dehydrogenative transformation is considered to be
a thermodynamically uphill process at ambient conditions. As
a result harsh reaction conditions or sacrificial hydrogen
acceptors are typically required to achieve a successful
dehydrogenation. So far, dehydrogenations often proceed in
the presence of metal complexes with an external oxidant
such as O2 and stoichiometric metal oxidants which result in
limited selectivity and poor functional group tolerance. Thus,
finding a versatile oxidative dehydrogenation procedure of
simple unactivated N-heterocycles in the absence of external
oxidants, particularly for pyrrole synthesis is challenging task.
Functionalized pyrroles display a wide variety of interest-
ing biological activities including anti-tumor, anti-inflamma-
tory, and anti-HIV activities.[2] In addition, their role as
intermediates in the synthesis of dyes,[3] flavoring compo-
nents,[4] and organic functional materials[5] have been estab-
lished. As a result, the synthesis of substituted pyrroles has
received attention from the synthetic and medicinal chemists.
In this regard, the synthesis of b-substituted pyrroles is of
particular interest as current procedures mainly involve
multistep routes and stoichiometric derivatization steps that
limits their applicability.[6] Therefore, advancement in the
direct synthesis of b-substituted pyrroles via an oxidative
dehydrogenative approach is in great demand.
Scheme 1. C(sp3)-H functionalization and aromatization-sulfonylation
strategy.
In recent years, visible-light photoredox catalysis has
evolved as an alternative to conventional multistep routes.
During our work on the photoredox-catalyzed a-func-
tionalization[11] of amines, we noticed that the reaction of N-
substituted pyrrolidines with sulfonyl chlorides resulted in the
formation of small amounts of pyrrole. This led us to examine
the aromatization reaction in more detail. The reaction of 1-
benzylpyrrolidine 1a and p-toluenesulfonyl chloride 2a in the
presence of Ir[dF(CF3)ppy]2(dtbbpy)PF6 A in acetonitrile in
nitrogen atmosphere and under visible-light irradiation at RT
gave the C3-sulfonylated pyrrole 3a in 14% yield (Table 1,
entry 1). To our surprise, neither traces of C(sp3)-H sulfony-
lation product nor C2-sulfonylated pyrrole were observed in
this new dehydrogenative aromatization sulfonylation reac-
tion. Subsequently optimization studies were carried out and
[*] Dr. K. Muralirajan,[+] Dr. R. Kancherla,[+] Prof. Dr. M. Rueping
KAUST Catalysis Center (KCC)
King Abdullah University of Science and Technology (KAUST)
Thuwal, 23955-6900 (Saudi Arabia)
E-mail: magnus.rueping@kaust.edu.sa
Prof. Dr. M. Rueping
Institute of Organic Chemistry, RWTH Aachen University
Landoltweg 1, 52074 Aachen (Germany)
Magnus.Rueping@RWTH-Aachen.de
[+] These authors contributed equally to this work.
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
https://doi.org/10.1002/anie.201808427.
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Table 1: Evaluation of reaction conditions.[a]
ates) and addition of water (to improve the solubility, Table 1,
entry 6). The use of different Ir, Ru and organic photo-
catalysts, however, did not improve the yield of the desired
product. Control experiments demonstrated that the presence
of photocatalyst, base, and visible light are all necessary to
attain the 3-substituted pyrrole (Table 1, entry 11).
Entry
Change in given reaction conditions[a]
Yield[b] (3a,%)
N-Benzyl-substituted pyrrolidine can be applied in this
transformation. This is a rather unexpected result since the
formation of the more stable benzyl radical and subsequent
formation of benzaldehyde via the iminium ion could be
anticipated. Having a suitable and viable catalyst system, we
decided to explore the scope of C3-functionalizing dehydro-
genation reactions (Table 2–4). A series of electronically
diverse arylsulfonyl chlorides ranging from electron-rich (4-
OMe, 4-nBu, and 4-tBu) to electron-deficient groups (4-
halides and 4-phenyl) delivered good yields of the desired C3-
sulfonylated pyrroles (Table 2).
1
2
3
4
5
6
7
8
acetonitrile
propionitrile
isobutyronitrile
2-methylbutyronitrile
14
20
18
27
65
85
12
8
additive: KPF6 (1 equiv)
additive: KPF6 (1 equiv)+H2O (3 equiv)
Ru(bpz)3(PF6)2 instead of Cat. A
Ru(bpy)3(PF6)2 instead of Cat. A
Ir[(ppy)2(dtbbpy)]PF6 instead of Cat. A
[Mes-Acr-Me]ClO4 instead of Cat. A
without photocatalyst/base/light
9
10
11
20
22
0
Further efforts to extend the scope of the dehydrogen-
ation sulfonylation reaction focused on the variation of the N-
substitution of the pyrrolidines (Table 3). N-Alkyls bearing
primary, secondary, tertiary or ester containing substrates 1b–
e provided the corresponding products. However, the N-
methyl and N-ethyl substrates react slowly which may be due
to the instability of the intermediary formed radical. Never-
theless, N-alkyl groups including tert-butyl acetate, ethyl 2-
[a] Reaction conditions: 1a (0.2 mmol), 2a (0.6 mmol), Cat. (2 mol%),
Cs2CO3 (2.5 equiv), additive (1 equiv), solvent (0.1m), N2. [b] Deter-
mined by 1H-NMR analysis of the crude mixture relative to trimethoxy
benzene as an internal standard.
reaction parameters including solvents, additives, and amount
of reactants were also examined (see the Supporting Infor-
mation). The comprehensive optimization revealed the need
of a base, counter anion (to stabilize iminium ion intermedi-
Table 3: Scope of N-alkyl variation.[a]
Table 2: Scope of the sulfonyl variation.[a]
[a] Reaction conditions: 1b–x (0.2 mmol), 2a (0.6 mmol), Ir[dF-
(CF3)ppy]2(dtbbpy)PF6 (2 mol%), Cs2CO3 (2.5 equiv), KPF6 (1 equiv),
H2O (3 equiv), and 2-methylbutyronitrile (0.1m) by blue LED irradiation
(34 W) with fan cooling for 30 h in N2; isolated yields are given.
[b] Reaction time 40 h.
[a] Reaction conditions: 1a (0.2 mmol), 2a–l (0.6 mmol), Ir[dF-
(CF3)ppy]2(dtbbpy)PF6 (2 mol%), Cs2CO3 (2.5 equiv), KPF6 (1 equiv),
H2O (3 equiv), and 2-methylbutyronitrile (0.1m) by blue LED irradiation
(34 W) with fan cooling for 30 h in N2; isolated yields are given.
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methyl propanoate and 1-(1-phenylpropan-2-yl) give compa-
rable results to the benzyl-protected derivatives. The yield
gradually improved by introducing N-benzyl or ester groups
at the a-position (4 f,g, 74–85% yield). This observation is in
good agreement with the notion that the generation of radical
from benzyl substituted amine is more favorable than
aliphatic substituted amine.[12] In addition various benzyl
groups regardless of the electronic nature of the aryl rings
(4i–t, 54–89% yield) could efficiently be applied. Pyrrolidines
with 2-methyl and 2-methyl pivalate substitutions also
provided the corresponding C3-sulfonylated product 4u
(64:36) and 4v (91:9) in 61 and 45% yield, respectively with
moderate to good regioselectivity. The reaction of methyl (4-
nitrobenzyl)prolinate amino acid with 2a provided highly
regioselective C4-sulfonylated pyrrole 4w (99:1) in 40% yield.
The lower yield may be explained by the formation of the
more stable tertiary carbon centered radical which impedes
the second electron oxidation to form the iminium ion
intermediate. Importantly, benzene fused pyrrolidine could
be applied in this protocol and afforded 1-benzyl-5-tosyl-1H-
indole 4x.[13]
Finally, we extended the scope of the dehydrogenative
aromatization sulfonylation to N-aryl substituted pyrrolidines
(Table 4). The arenes bearing methyl or methoxy function-
ality at the 2,4,6-positions underwent reaction with good
Table 4: Scope of N-aryl variation.[a]
Scheme 2. Mechanistic studies and applications.
This supports that the reaction might proceed through the
radical pathway. We also performed on-off light switching
experiments. The reaction mixture was irradiated for
30 minutes and subsequently the light source was removed
and the reaction continued for 5 h in the dark. While initially
34% yield (up to 30 minutes) was formed in the presence of
light, no further product formation was observed in the dark,
indicating that a long radical-chain mechanism is not
involved. Moreover, we measured the quantum yield (F =
0.249) of the reaction (see the Supporting Information)
confirming that the reaction is not involving a photoinitiated
chain propagation. Finally, Stern–Volmer photoluminescence
quenching experiments (see the Supporting Information)
revealed that the excited-state *Ir3+ photocatalyst A is
quenched more readily by the amine substrate, indicating
a reduction quenching cycle.
Importantly, if the reaction was stopped after one hour 1-
mesityl-4-tosyl-2,3-dihydro-1H-pyrrole (6a-1) was isolated as
the major product in 68% yield along with 7% of 6a
(Scheme 2b). This is an important result indicating that
1) during the reaction 2,3-dihydro-1H-pyrrole D (Scheme 3)
is formed which further reacts with the sulfonyl radical to
form 6a-1, 2) no traces of sp3-sulfonylation product was
observed demonstrating that a possible sp3-sulfonyl amino
radical–radical cross-coupling is not occurring under the
applied reaction conditions. Similarly, we conducted the
reaction of 6a-1 with 2a (1 equiv) to confirm that 6a-1 acts
as intermediate. The reaction proceeded successfully and
[a] Reaction conditions: 5a–e (0.2 mmol), 2a (0.6 mmol), Ir[dF-
(CF3)ppy]2(dtbbpy)PF6 (2 mol%), Cs2CO3 (2.5 equiv), H2O (3 equiv), and
2-methylbutyronitrile (0.1m) by blue LED irradiation (34 W) with fan
cooling for 15 h in N2; isolated yields are given.
efficiency (6a–c, 79–93%). The presence of electron with-
drawing group in ortho position provided 6d in lower yield,
however, this effect was offset by the introduction of donating
group in arene ring (6e, 68%). Additionally, this dehydrogen-
ative aromatization process was also found feasible in gram-
scale reaction (6a, 86% yield).
À
In this newly developed reaction, multiple C H activation
and SET steps are involved to complete the aromatization. In
order to understand the reaction mechanism, we performed
different control reactions (Scheme 2). First, the radical
trapping agent TEMPO was added under the standard
reaction conditions (Scheme 2a). As expected the reaction
was quenched and no dehydrogenative product was observed.
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a reductant [E1/2ox = À1.37 V vs. SCE in CH3CN] to reduce 4-
methylbenzenesulfonyl chloride 2a [E1/2 = À0.94 V vs. SCE in
CH3CN][16] to corresponding sulfonyl radical (2a’) and
regenerates Ir3+ via SET. Regioselective sulfonyl radical
addition to the enamine D provides the carbon-centered
radical intermediate E, which gets further oxidized to form
6a-1. Finally, aromatization and product formation of 6a
occurs in sequence analogous to the formation 6a-1 form 1.
To fulfill the catalytic cycle arylsulfonyl chloride acts addi-
tionally as a sacrificial electron acceptor to form isolatable S-
(p-tolyl) 4-methylbenzenesulfonothioate 2a-1 and 1,2-di-p-
tolyldisulfane 2a-2 (see reduction cycle).[17] However, the
exact mechanism for formation of 2a-1 and 2a-2 from 2a
needs to be established.
In summary, an unprecedented dehydrogenative aroma-
tization sulfonylation reaction of pyrrolidines was achieved
under photoredox catalysis with the use of sulfonyl chlor-
ides[18] which have a dual function as catalyst regeneration and
sulfonylation reagent. The isolation of the intermediates
provided insight into the reaction mechanism of this new
aromatization protocol. N-Alkyl pyrrolides and in particular
N-benzyl-protected derivatives can be effectively applied and
that the possible formation of exocyclic imines resulting in
aldehydes or ketones cannot be observed. The full dehydro-
genation of saturated heterocycles is a topical area in
synthesis as well as in hydrogen storage and release but
typically requires the use of transition metals and/or higher
temperatures. The new protocol, however, allows the dehy-
drogenative aromatization of pyrrolidine without the need for
additional transition metals or oxidants; proceeds at room
temperature; and thus exhibits excellent functional group
tolerance. Moreover, the regioselective sulfonylation pro-
vides b-substituted sulfonyl pyrroles which are otherwise
difficult to access and which can be used in the synthesis of
new nitrogen/sulfon-fused, ladder-type, p-conjugated mole-
cules. Further exploration of this new methodology by
exchanging the individual components will result in the
dehydrogenative synthesis of regioselectively substituted
heterocylces.
Scheme 3. Proposed catalytic cycle for dehydrogenative aromatization-
sulfonylation reaction.
gave 6a in 97% yield. Furthermore, a reaction with N,N-
dimethyl benzyl amine (Scheme 2c) as substrate was carried
out in order to examine whether the C(sp3)-H sulfonylation
can occur. However, no sulfonylation product was observed
indicating that a radical–radical cross-coupling is not occur-
ring. An explanation for this observation could be absence of
a persistent radical. In addition, neither C3 sulfonylation nor
Friedel–Crafts sulfonylation were observed if the pyrrole was
reacted under our reaction conditions (Scheme 2d). Finally,
the origin of this dehydrogenation sequence can be rational-
ized through iminium ion formation from a-amino radical,
À
either through direct C H abstraction from the a-positions or
a sequential deprotonation–oxidation process.[14] In order to
explore the synthetic utility of these reaction products,
debenzylation reaction was carried out which gave 3-tosyl-
pyrrole 7 in 76% yield, providing the opportunity for further
functionalization (Scheme 2e). Furthermore, an unusual
nitrogen/sulfone fused a ladder-type p-conjugated molecule
8 was synthesized by Pd-catalyzed intramolecular dehydro-
genative cross-coupling method (Scheme 2 f). The isolated
red color compound 8 possibly acts as organic functional
material with considerable red-shift absorptions in the UV/
visible range.[15]
Acknowledgements
Based on these observations a plausible mechanism for
the oxidative dehydrogenation reaction is proposed
(Scheme 3). The irradiation of photoredox catalyst Ir[dF-
The authors acknowledge KAUST for financial support. The
research leading to these results has received funding from
the European Research Council under the European Unionꢀs
Seventh Framework Programme (FP/2007-2013)/ERC grant
agreement number 617044 (SunCatChem).
(CF3)ppy]2
(dtbbpy)PF6
[dF(CF3)ppy = 2-(2,4-difluoro-
phenyl)-5-trifluoro-methylpyridine,
dtbbpy = 4,4’-di-tert-
butyl-2,2’-bipyridine, hereafter represented as Ir3+) promotes
metal-to-ligand charge transfer (MLCT) followed by inter-
system crossing (ISC) to generate excited *Ir3+ by initial
red
abstraction of a photon from visible light. This *Ir3+ [E1/2
=
Conflict of interest
1.21 V vs. SCE in CH3CN] act as an oxidant to oxidize the
amine substrate via a single electron transfer (SET) mecha-
nism. This single electron transfer step can generate a reduced
iridium complex (Ir2+) with successive formation of the amine
radical cation A and a-amino carbon radical B through
The authors declare no conflict of interest.
Keywords: aromatization · C(sp3)-H functionalization ·
C3 sulfonylation · dehydrogenation · redox chemistry
À
deprotonation of the amino a-C H. The following SET
provides iminium ion C which upon further tautomerization
leads to the stable endo Z-enamine D. The reduced Ir2+ acts as
4
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Manuscript received: July 23, 2018
Revised manuscript received: August 22, 2018
Version of record online: && &&, &&&&
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Communications
Photoredox Catalysis
K. Muralirajan, R. Kancherla,
M. Rueping*
&&&& — &&&&
Dehydrogenative Aromatization and
Sulfonylation of Pyrrolidines: Orthogonal
Reactivity in Photoredox Catalysis
Functionalized pyrrolidines: A protocol
for the oxidative dehydrogenative aroma-
tization and selective sulfonylation of N-
heterocycles under visible-light photore-
dox catalysis was established, whereby
arylsulfonyl chlorides were found to be
both catalyst regeneration and sulfonyla-
tion reagents.
6
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Angew. Chem. Int. Ed. 2018, 57, 1 – 6
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